Membranes which possess a microporous, open-celled structure are not new. For example, methods of making microporous membranes have been proposed whereby a crystalline elastic starting film is drawn or stretched at ambient temperatures (i.e., so-called "cold drawing") in an amount of about 10 to about 300 percent of the starting film's original length, with subsequent stabilization by heat setting of the drawn film under tension such that the film is not free to shrink or can shrink only to a limited extent. An example of such a "cold drawing" process is U.S. Pat. No. 3,426,754 issued to H. S. Bierenbaum et al on Feb. 11, 1969.
Another technique employed in the art of making microporous membranes is the so-called "solvent stretch" process as exemplified by U.S. Pat. Nos. 4,255,376 issued to J. W. Soehngen on Mar. 10, 1981 and 4,257,997 issued to J. W. Soehngen et al on Mar. 24, 1981. Briefly, the solvent stretch process involves preparing microporous films from a two-component precursor film (i.e., one having an amorphous component and a crystalline component). The precursor film is brought into contact with a swelling agent and longitudinally stretched while still in contact with the swelling agent. Subsequently, the swelling agent is removed while the film is maintained in its longitudinally stretched condition to render the film microporous.
Corona discharge treatments have also been employed in the past to render polymeric films microporous as exemplified by U.S. Pat. No. 3,880,966 issued to D. Zimmerman et al on Apr. 29, 1975. In this conventional process, polymeric films are subjected to a corona discharge treatment so as to render the film permeable. The films are then rendered microporous by stretching and heat setting.
Other processes for producing open-celled microporous membranes using sequential "cold" and "hot" stretching steps have also been proposed as exemplified by U.S. Pat. Nos. 3,679,538 issued to M. L. Druin et al on July 25, 1972 and 3,801,692 issued to D. Zimmerman on Apr. 2, 1984, the entire content of these prior-issued patents being expressly incorporated herein by reference. Generally these processes include the steps of cold stretching a non-porous, crystalline, elastic film, thereafter hot stretching the cold stretched film to render it microporous, and finally heat setting the microporous film.
Special techniques have also been proposed for the sequential "cold" and "hot" stretching process. For example, in U.S. Pat. No. 3,843,761 issued to H. S. Bierenbaum et al on Oct. 22, 1978 (the entire content of which is expressly incorporated herein by reference), a process is disclosed whereby an annealed film is initially cold stretched and then subsequently subjected to multiple hot stretching steps so as to render a variety of polymeric films microporous. According to U.S. Pat. No. 4,138,459, another technique is disclosed whereby polymeric films are rendered microporous by subjecting an annealed film to cold stretching, hot stretching, and heat relaxing steps.
Pore density is an important physical attribute of a microporous membrane since it directly determines the gas flux of the membrane (i.e., permeability). That is, the greater density of the pores in the microporous membrane, the greater the ability of the film to allow a volume of gas to flow through a fixed surface area of the membrane in a fixed period of time. Such permeabilities are usually expressed in terms of "Gurley Values" (sometimes also referred to as "Gurley Seconds"), which is the time required for 10 cm.sup.3 of air to pass through 1 in.sup.2 of membrane from one exterior surface to an opposite exterior surface thereof when a pressure differential of 12.2 inches of water is applied across the membrane. Since permeability is a measure of the ease of mass transfer across the membrane, lower Gurley Values correspond to lower mass transfer times and hence correspond to higher permeabilities and a concomitant greater ease of mass transfer.
The capability of membranes to have a greater ease of mass transfer thereacross is important in many end use applications, such as filter media, solute extraction membranes, blood oxygenation membranes, battery separators, etcetera. However, since the pore density of microporous membranes (and its resulting permeability) is a function of pore size, in order to increase the pore density of a microporous membrane, the pore size must be correspondingly reduced. While the prior art processes noted above do permit microporous membranes to be produced which have satisfactory permeability properties, there still exists a continual need for improvements.
According to the present invention, novel open-celled microporous membranes are provided having increased pore densities and correspondingly reduced pore sizes as compared to conventional microporous membranes. Surprisingly, the novel membranes of the present invention are produced by subjecting a membrane precursor to an increased cold stretch which is preferably accomplished in multiple discrete uniaxial cold stretching steps prior to hot stretching. That is, the total amount of cold stretching of the membrane precursor is increased as compared to the cold stretching employed in conventional microporous membrane processes. And, this increased cold stretching is preferably distributed over multiple discrete cold stretching steps prior to hot stretching. Advantageously, the total cold stretch employed according to the present invention elongates the membrane precursor greater than about 30%, and more advantageously greater than about 40%, based upon its initial length prior to cold stretching, this total cold stretch preferably being distributed over two or more (preferably two to four) discrete sequential cold stretching steps.
The membranes of the invention will also advantageously exhibit a pore morphology characteristic of conventional membranes obtained by sequential uniaxial cold and hot stretching techniques. That is, the membranes of this invention will have a plurality of elongated, non-porous, interconnecting surface regions which have their axes of elongation substantially parallel to each other, and substantially normal or perpendicular to the direction in which the membrane is stretched. Substantially alternating with and defined by the non-porous surface regions is a plurality of elongated, porous surface regions which contain a plurality of parallel fibrils. The fibrils are connected at each of their ends to the non-porous regions and are substantially perpendicular to them. A dense plurality of pores is thus defined between these fibrils.
The membranes of the present invention are open-celled and are particularly characterized by a reduced bulk density as compared to a corresponding membrane precursor having a nonopen-celled structure. Moreover, the membranes of this invention exhibit permeabilities (as determined by a gas flux of less than about 22 Gurley seconds), and an average pore density of greater than about 75 pores per square micron of membrane surface. The pores will typically have an average length as measured in the direction of the longitudinal stretching of less than about 0.10 micron (advantageously between about 0.05 to 0.09 micron), an average pore breadth as measured in a direction perpendicular to the longitudinal stretching of less than about 0.035 micron (advantageously between about 0.024 to 0.035 micron), an average pore surface area of less than about 2.5.times.10.sup.-3 square micron (advantageously between about 0.9.times.10.sup.-3 to 2.5.times.10.sup.-3 square micron), an average pore radius as determined by mercury porosimetry of less than about 0.040 micron (advantageously between about 0.0365 to 0.040 micron), and a specific surface area of greater than about 45 m.sup.2 /g (i.e., as determined by BET analysis using a Quantasorb.TM. apparatus manufactured by the Quantachrome Corporation--see also, S. Brunauer et al, Journal of American Chemical Society, vol. 60, pg. 309 (1938); and F. M. Nelson et al, Analytical Chemistry, vol. 30, pg. 1387 (1958), the entire content of each of these articles being expressly incorporated hereinto by reference, for a further discussion of BET analysis).
Moreover, the membranes of the present invention are translucent and exhibit a characteristic "bluish" color hue. Specifically, the membranes of this invention exhibit a b.sup.* value of less than -10, and more specifically between about -11 to about -14 when a single ply membrane sample is analyzed in a Macbeth Coloreye.TM. apparatus against a black background. The reasons for this "bluish" color hue are not fully understood at this time. However, without wishing to be bound to any particular theory, it is surmised that it is caused by the increased pore density and decreased pore size which apparently cause different diffusion and scattering of light in the visible spectrum as compared to conventional opaque membranes of similar geometry. This different scattering of light thus apparently translates into a visibly perceptible bluish color hue being imparted to the membranes of this invention.
The microporous membranes of the invention may be produced in film or fiber (e.g., hollow fiber) form and used in their as-produced form or further configured so as to suit a particular end use application in which it may be employed. For example, the membranes of the present invention may initially be in a hollow fiber form and then subsequently sliced longitudinally so as to then be in the form of a film. Thus, although reference has been, and will hereinafter be made to the present invention being embodied in the form of a film, this is merely for ease of explanation and should be considered as being equally applicable to hollow fiber or other physical membrane forms.